Using variable pupil filters to optimize the resolution in multiphoton and saturable fluorescence confocal microscopy.

Pupil filters are widely used to improve the resolution of confocal microscopes. We analyze the possibilities of applying them to N-photon microscopy. We find that taking a linear combination of images obtained with several pupil filters can improve the resolution by a factor of N (compared to a conventional microscope). When applied to saturable fluorescence, this technique allows one to observe fluorescent objects with, in principle, unlimited spatial resolution.